Unboxed, primitive, multidimensional tables. In instance for Ix-type
keys comes with the package. This package trades safety for speed. The index
operator (!) is basically the only function that does bounds-checking and
only with an assertion. This, however, is by design. The only way to get an
immutable table from a mutable one is by the unsafeFreezeM operation.
Again, it is by design that both data structures share the same memory
pointer internally.